1. Field of the Invention
The present invention relates to a rotary anode type X-ray tube and, more particularly, to an improvement in a rotating mechanism for supporting a rotaryanode of the X-ray tube.
2. Description of the Related Art
As is known, in a rotary anode type X-ray tube, a disk-like anode target is supported by a rotary structure and a stationary shaft having a bearing portion therebetween, and an electron beam emitted from a cathode is radiated on the anode target while the anode target is rotated at a high speed by a rotating magnetic field generated by energizing the electromagnetic coil of a stator arranged outside a vacuum envelope, thus irradiating X-rays. The bearing portion is constituted by a rolling bearing, such as a ball bearing, or a dynamic pressure type sliding bearing which has bearing surfaces with spiral grooves and uses a metal lubricant consisting of, e.g., gallium (Ga) or a gallium-indiumtin (Ga-In-Sn) alloy. Rotary-anode type X-ray tubes using the latter bearing are disclosed in, e.g., Published Examined Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Application Nos. 60-97536, 60-117531, 61-2914, 62-287555 and 2-227948.
The rotary structure for supporting the anode target usually includes a rotating shaft fixed to the anode target and made of metal having a high melting point, a cylindrical core fixed to the rotating shaft and made of ferromagnetic matter such as iron to serve as a rotor for the induction motor, and an outer cylinder fitted onto and welded to the cylindrical core and made of metal such as copper having a high conductivity. The rotary structure is rotated at high speed on the principle of the induction motor while applying rotating magnetic field from a stator located outside the tube to the rotating structure.
In the rotary anode type X-ray tubes which are disclosed in the above-mentioned Official Gazettes, molybdenum, molybdenum alloy, tungsten or tungsten alloy is used as material for forming the slide bearing surfaces. When the bearing surfaces are made of one of these metals, however, there is fear that the bearing surfaces are likely to be oxidized at the processes of manufacturing the X-ray tube and that their wet capability relative to the liquid metal lubricant is degraded. Further, the bearing surface and the liquid metal lubricant may be reacted with each other and the metal lubricant may be permeated into the bearing surface at high temperature, when the X-ray tube is heated in a manufacturing process or during an operation of the X-ray tube. Thus, the bearing surfaces may be made rough and changed in dimension. The dimension of a clearance between the bearing surfaces is thus changed, so that stable bearing work cannot be kept.